Optical bar code scanning is widely used in a variety of applications. In the typical operation of a bar code scanner, numerous sources of potential error are present. The decoding electronics of a scanner typically receive and interpret a scan signal having as a primary component the actual signal produced as a scan beam moves across light and dark areas of a bar code. The voltages of the scan signal, and the changes in the voltage as the signal achieves positive and negative peaks, are interpreted in order to identify positive and negative transitions. However, noise components are typically present that affect the scan signal and its interpretation. Noise sources may include contributions such as baseline noise inherent in a particular scanner, optical noise such as paper noise produced by characteristics of a bar code label, optical characteristics of various components of the scanner, such as mirrors, window surfaces and other components, optical and electrical characteristics of an environment in which the scanner is used, and other components. Noise may affect the scan signal, adding to or diminishing the scan signal voltage. If such addition or diminishing occurs at critical times, noise may produce a false identification of a peak or failure to recognize a peak.
A typical scan of a bar code involves the intersection of a bar code by one or more scan lines of a scan pattern. The bar code may be completely transected by some scan lines, partially transected by others, and not transected at all by still others. Scan lines that partially transect a bar code may produce a scan signal reflecting incomplete or incorrect data, and scan lines that do not transect a bar code at all may produce a signal reflecting incorrect data, or simply noise. Incorrect data must be rejected in order to achieve accurate results.
The photosignal produced by a scanner regularly undergoes abrupt changes. To take a common example, when no object is within a scan zone of a scanner, the scan signal essentially reflects noise. The signal to noise ratio of the scan signal is therefore less than 1. When an object is introduced into the scan zone and a bar code is intersected by a scan pattern, the signal to noise ratio abruptly changes to a value greater than 1. Immediately around this transition, the probability of a decode error is increased. Numerous other sources of noise and errors may be present.